This invention relates generally to metallic particles and films, and more particularly to methods for their production by linking the electron pumping features of certain biological systems, such as the photosynthetic machinery, with the reductive precipitation of metallic particles.
Photosynthesis is the biological process that converts electromagnetic energy into chemical energy through light and dark reactions. In green algae and higher plants, photosynthesis occurs in specialized organelles, called chloroplasts. The chloroplast is enclosed by a double membrane and contains thylakoids, consisting of stacked membrane disks (called grana) and unstacked membrane disks (called stroma). The thylakoid membrane contains two key photosynthetic components, photosystem I and photosystem II, designated PSI and PSII, respectively, as depicted schematically in FIG. 1. During photosynthesis, water is split into molecular oxygen, protons and electrons by PSII. Electrons derived from the splitting of water molecules are transported through a series of carriers to PSI where they are further energized by a light-induced photochemical charge separation and transported across the thylakoid membrane where they are used for the enzymatic reduction of NADP+ to NADPH. This biological reaction is further utilized for chemical energy production, primarily in the form of ATP.
Ultrafine metallic particles, e.g., nanoparticles, are important precursors for use in the fabrication of advanced material structures, such as thin continuous films. Conventionally, metallic films have been deposited on substrates by methods such as chemical vapor deposition (CVD), sputtering, plating, and the like. Unfortunately, such methods do not generaly offer a degree of control desired for the deposition of nanostructured materials, e.g., films having nanometer range thicknesses or grains. Therefore, a method which could drive the nucleation, growth and deposition of nanoparticles in a quantitative, rapid, and energy-efficient manner would be highly desirable for many applications, including materials processing, catalysis, separations, electronics, energy production processes, and environmental applications.
Despite the extensive investigation concerning the photosynthetic machinery, the use of photosynthesis-related principles for materials synthesis and processing has not been described. The present invention, by exploiting the electron pumping characteristics of the photosynthetic machinery for nanoparticle production and processing applications, provides improved methods and materials which overcome or at least reduce the effects of one or more of the aforementioned problems.
This invention broadly concerns methods for the controlled deposition of ultrafine metallic particles and thin films via biomolecular electronic mechanisms. In particular, the invention takes advantage of the electron-pumping characteristics of photosynthesis system I (PSI), and other biological systems having similar features, for photocatalytically reducing metal precursor chemicals into metallic nanostructured materials.
Therefore, according to one aspect of the invention, a metallic film is formed by providing a liquid suspension which is at least partly comprised of a plurality of photosystem I-containing units, metal precursors, and any other component necessary or desired for effecting the photochemical reaction on the PSI-containing unit, e.g., electron donor molecules. The liquid suspension is contacted with light, preferably in the form of intermittent flashes, under conditions effective for causing the controlled reductive precipitation of the metal precursors on the photosystem I-containing units to form photosystem I-metal complexes. Generally, the liquid suspension containing the photosystem I-metal complexes is provided above the surface of a solid or semisolid substrate, such as a surface comprised of gold, silicon, silica, alumina, zirconia, titania, or any of a variety of other materials. Thereafter, the liquid of the liquid suspension is removed, for example by applying heat and/or vacuum to evaporate the liquid. Upon removal of the liquid, a film is thereby formed on the surface of the substrate that is at least partly comprised of the metal from the photosystem I-metal complexes.
In another aspect of the invention, a plurality of PSI-containing units may be anchored or otherwise coated on a desired substrate prior to performing the photo-induced formation of the photosystem I-metal complexes. This PSI-coated substrate is then contacted with a solution containing a plurality of metal precursors, electron donor molecules and other desired components. The solution and the underlying PSI-coated substrate are thereafter contacted with light energy under conditions effective for causing the reductive precipitation of the metal precusor on the photosystem I-containing unit to form photosystem I-metal complexes that are spatially constrained along the surface of the substrate. Under appropriate reaction conditions, the metal particles on the PSI-containing units are controllably grown to a size at which metal particles on adjacent PSI-containing units above the substrate merge into a continuous metallic film.
In another aspect of the invention, metallic nanoparticles are provided by forming PSI-metal complexes in a suitable liquid suspension and thereafter separating the metal particles from the PSI-metal complexes. The means by which the metal particles are separated may include any suitable chemical, physical or mechanical treatment sufficient to remove the particles from the complexes without adversely affecting their chemical composition or structural integrity.
The methods of the present invention offer numerous advantages over other technologies, e.g., CVD, sputtering, electroless plating, MBE, etc., for the production of metallic particles, films, and other materials such as alloys and composites. First the methods allow for precisely controlled metal particle nucleation and growth for atomic-level deposition. The methods are energy-efficient and have no requirement for high temperature or pressure/vacuum systems, such as are required for other technologies. Moreover, the methods offer controllable deposition kinetics which may be varied through modulation of the light energy input level. Finally, the methods are environmentally benign and non-interfering, i.e, light is the controlling mechanism.
The nanosized particles of this invention, and the products derived therefrom, will support a broad range of applications, including energetics (e.g., as fuel in propellants), explosives, microelectronics, catalysis, powder metallurgy, coating and joining technologies, and others. For example, for catalysis/separations applications, reductions in metallic film thicknesses will reduce metal cost, allow higher hydrogen flux, enhance permselectivity, and improve membrane reactor efficiency. The membrane reactors have been used in energy generation and environmental application processes, such as the advanced power generation and environmental application processes, such as the advanced power generation systems-integrated gasification combined cycle (IGCC) systems.
In the petrochemical industry, important applications may include hydrogen separation and membrane reactions concerning hydrocarbon (such as propane and ethylbenzene) dehydrogenation and natural gas steam reforming (e.g., CH4+HzOxe2x86x92CO+3H2), oxidative reforming of methane to syngas, and partial oxidation or oxidative coupling of methane into hydrogen and higher hydrocarbons. For these chemical reactions, palladium (Pd)-based membranes may be preferred in terms of temperature resistance and hydrogen permeability, however other metals, e.g., platinum and osmium, may also be used. In addition, metallic nanoparticle arrays of uniform particle size in the range of about 2.5-100 nm deposited over a large area oxide (1 cm2) support offer promising alternatives to single crystal surface catalysts.
The methods of the invention also find use in a variety of applications involving electronic materials and devices, such as electronic circuit board fabrication, metallic (Pd) buffer layer preparation for superconducting RABiTS (Rolling-Assisted Biaxially Textured Substrate), and multilayer devices (such as hard disk reading head memory chip) based on GMR (Giant Magnetorresistance).